Bis(thiosemicarbazone) complexes of transition metals have been known for nearly 50 years, and their biological activity was recognised early on. Despite demonstrations of for instance their cytotoxic, antibacterial, antimalarial, antiviral, antitumour and antifungal activity, the mechanism of their action at the cellular level remains completely unknown, although it is clear that complexes are more active than the free ligands.
There are several radionuclides of copper that have the potential to be used as radiotherapeutic or diagnostic imaging agents. For example, copper-64 (64Cu) decays via electron capture, positron, beta and Auger emissions, which means that it can be used for both PET imaging and radiotherapeutic applications. In one embodiment the present invention relates to the development of copper radiopharmaceuticals in the form of suitable stable chelators that can be readily functionalized with appropriate biomolecules to provide stable copper bioconjugates.
The use of functionalised diazenide (NNAr, Ar=aryl or 2-pyridyl) ligands for the targeting of technetium radiopharmaceuticals particularly for the pyridyl (HYNIC) system is known. The HYNIC bifunctional chelate system is extremely versatile and has been used to conjugate technetium to chemotactic peptides, somatostatin analogues, liposomes and a folate receptor ligand. There are a number of technetium based systems in clinical trials for imaging, but the system is limited to technetium (and to a lesser extent rhenium) since these have high oxidation metal precursors which can form robust metal-nitrogen multiple bonds. The system is not suitable for use with, for example, copper radionuclides.
Most research into bifunctional chelates for targeted radiopharmaceuticals with copper isotopes has focused on the use of tetra-aza based macrocyclic ligands such as 1,4,8,11-tetraazacyclotetradecane-N,N′,N″,N′″-tetraacetic acid (TETA). Despite this success and the high stability of copper-TETA complexes there is still significant loss of the radionuclide from the chelate due to transchelation with the protein superoxide dismutase.
In U.S. Pat. No. 5,843,400 Fujibayashi et al describe diagnostic agents for hypoxia or mitochondrial dysfunction, which comprise radioactive copper complexes, which agents have good transferability to the target tissue, reduction reaction affinity at a hypoxic site, high stability in a non-target site and rapidly disappear.
Z.-M. Xue, Y.-P. Tian, D. Wang and M. H. Jiang describe in Dalton. Trans., 2003, 7, 1373 the fluorescence of a bis(thiosemicarbazone) complex of zinc. Cowley et al in Chem. Commun., 2005, 845-847 describe the fluorescence of a bis(thiosemicarbazone) complex of zinc with a fifth apical chlorine ligand.